Two pulses cross 1.00 m at once: one through vacuum, one through a material of your choice. Raise the index n and watch light slow to v = c ÷ n, its wavefronts bunch up, and the medium pulse fall behind.
Speed in medium v = c ÷ n
2.00 × 108 m/s
66.7% of the speed of light
Wavelength in medium = vacuum ÷ n
393 nm
in vacuum: 589 nm · colour unchanged
Time to cross 1.00 m t = n·L ÷ c
Through medium5.00 ns
Through vacuum3.34 ns
Closest real material
Crown glass (1.52)
Refractive index n1.50
Vacuum wavelength589 nm
Fixed: speed of light c = 299,792,458 m/s · path length L = 1.00 m
Tip: push n toward 2.42 and the medium pulse crawls — that is diamond, where light travels at barely 41% of c.